microwave-engineering

2National Institute of Technology, Warangal 29-08-2017
Microwave Engineering
Electromagnetic Spectrum

Microwave frequency range 1-30GHz
wave length 30cm-1cm

Microwave Frequency Range

1. Small size wavelength
f=1GHz
λ=c/f=3x1010/1x109=30cm
f=30GHz
λ=c/f=3x1010/30x109=1cm
Wave lengths are same as dimensions of components,
so distributed circuit elements or transmission
theory is applied.
Characteristics of Microwaves

Characteristics-Large Bandwidth
Large Bandwidth
High transmission rates used for communication
World’s data, TV and telephone communications are
transmitted long distances by microwaves between
ground stations and communications satellite

Characteristics-Line of sight
propagation

propagation

Characteristics-Transmission
Through Ionosphere

Through Ionosphere

Characteristics- Reflection From
Metallic Surfaces

Characteristics

Characteristics- Heating

Characteristics- Microwave
Resonance
Microwave Resonance: Molecular, atomic and nuclear
systems exhibit resonance when Present electromagnetic
Fields
Several resonance absorption lines are in microwave range

2329-08-2017Microwave Engineering
Application- Communications
National Institute of Technology, Warangal
Point to point communications
GSM 1.8 and 1.9 GHz
DVB-SH, 1.452, 1.492 GHz

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Wi-Fi
Wireless LAN networks 2.4GHz ISM band

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Wimax
Wimax(Worldwide Interoperability for Microwave Access)
2 to 11 GHz
PMP-Point to multipoint links

Wimax, WiFi

Satellite Communications
L band (1-2 GHz )Global Positioning System (GPS) carriers and
also satellite mobile phones, such as Iridium; Inmarsat providing communications at sea,
land and air; WorldSpace satellite radio.

Satellite Communications

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RADAR

RADAR
Radar is an object-detection system that uses radio waves to
determine the range, altitude, direction, or speed of objects.
It can be used to detect aircraft, ships, spacecraft, guided
missiles, motor vehicles, weather formations, and terrain.
Aviation
Marine
Meteorologists

Heating
Domestic Application: Heating, Microwave oven
Industrial Application: Food, Rubber, leather, chemical and
textile , pharmaceutical industries

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Remote Sensing
Remote sensing: Remote sensing is the acquisition of
information about an object or phenomenon without
making physical contact with the object and thus in
contrast to on site observation.

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Remote Sensing

Radio Astronomy
Radio Astronomy:
Radio astronomy is a
subfield
of astronomy
that studies celestial
objects
at radio frequencies.

Radio Astronomy
Arecibo 305 m ( about 20 acres) radio telescope, located in a natural valley in Puerto Rico.

Radio Interferometery
The Very Large Array, an interferometric array formed from many smaller telescopes

Medical Application

Microwave Imaging
Microwave imaging is a science which has been evolved from
older detecting/locating techniques (e.g., radar) in order to
evaluate hidden or embedded objects in a structure (or
media)using electromagnetic (EM) waves in microwave regime
(i.e., ~300 MHz-300 GHz)

Microwave Imaging
•concealed weapon detection at security check
points, structural health monitoring
•through-the-wall imaging.
•Disbond detection in strengthened concrete bridge
• Corrosion and precursor pitting detection in painted
aluminum and steel substrates
•Flaw detection in spray-on foam insulation

Industry Applications
 Microwave oven
 Drying machines – textile, food and paper industry for drying
clothes, potato chips, printed matters etc.
 Food process industry – Precooling / cooking, pasteurization /
sterility, hat frozen / refrigerated precooled meats, roasting of
food grains / beans.
 Rubber industry / plastics / chemical / forest product industries
 Mining / public works, breaking rocks, tunnel boring, drying /
breaking up concrete, breaking up coal seams, curing of cement.
 Drying inks / drying textiles, drying / sterilizing grains, drying /
sterilizing pharmaceuticals, leather, tobacco, power
transmission.
 Biomedical Applications ( diagnostic / therapeutic ) – diathermy
for localized superficial heating, deep electromagnetic heating
for treatment of cancer, hyperthermia ( local, regional or whole
body for cancer therapy).

41National Institute of Technology, Warangal 29-08-2017Microwave Engineering
Advantages
Large Bandwidth: It is very good advantage,
because of this, Microwaves are used for Point to Point
Communications.

Advantages
Better Directivity

Advantages
Better Directivity: At Microwave Frequencies, there are better
directive properties. This is due to the relation that as Frequency
Increases, Wavelength decreases and as Wavelength decreases
Directivity Increases and Beam width decreases. So it is easier to
design and fabricate high gain antenna in Microwaves.

Advantages
Small Size Antenna

Advantages
Low Power Consumption

Advantages
Low Power Consumption:The power required to transmit a high
frequency signal is lesser than the power required in transmission
of low frequency signals. As Microwaves have high frequency thus
requires very less power.

Advantages
Effect Of Fading
Space wave Sky wave

Advantages
Effect Of Fading: The effect of fading is minimized by using Line Of
Sight propagation technique at Microwave Frequencies. While at low
frequency signals, the layers around the earth causes fading of the
signal.
Space wave Sky wave

Fresnel Zone

Fresnel Zone
there should be no reflective objects in the 1st Fresnel zone
even Fresnel zone are out of phase with the direct-path wave
and reduce the power of the received signal
odd Fresnel zone are in phase with the direct-path wave and
can enhance the power

Limitations of Tubes at High
Frequencies

Vacuum tubes- Triode

Triode Amplifier Circuit

Limitations at Higher Frequencies
Inter electrode Capacitance

At frequencies greater than 1 GHz

Leads:Leads are used for physical support, to
transfer power and sometimes as a Heatsink.
Limitations at Higher FrequenciesLimitations at Higher Frequencies

Leads:Leads are used for physical support, to
transfer power and sometimes as a Heatsink.
In fact, any wires or component
leads that have current flowing
through them create magnetic fields.
When these magnetic fields are
created, they can produce an
inductive effect. Thus, wires or
components leads can act as
inductors if they are long enough

Parasitic Inductance and capacitance becomes very large
At Microwave frequencies
Limitations at Higher FrequenciesLimitations at Higher Frequencies

Reduce length of and area of leads, in turn reduces
Power handled.

Input conductance loads the circuitry, efficiency reduces.

Lead Inductance

Input Impedance
Input Voltage

Input Impedance
Input Current

Input Impedance
Input Admittance

Input Impedance

Input Impedance
Input Impedance

Input Impedance
Input Impedance
Input conductance loads the circuitry, efficiency reduces.

Gain Bandwidth

Gain Bandwidth
Gain bandwidth product is independent of frequency,
hence is constant. Hence resonant circuits are reentrant
or slow wave structures

Transit Time

Transit Time
•In the positive half-cycle, grid potential attracts the
electron beam and supplies energy to it

Transit Time
•In the negative half-cycle, it repels the electron beam
and extracts energy from it.

Transit Time
As a result, the electron beam oscillates back and
forth in the region between the cathode
and the grid, and may even return to the cathode.
The overall result is a reduction of the operating frequency
of the vacuum tube.

Transit Time
Reduce Transit Time
•Increasing the anode voltage
•Decreasing the inter-electrode spacing
However, the increase in anode voltage will
increase the power dissipation,
whereas the decrease in inter-electrode spacing
will increase the inter-electrode capacitance.

Transit Time
The increase in inter-electrode capacitance can be
reduced by reducing the area of the electrodes,
but this will reduce anode dissipation and hence the
output power.

RF Loss- Skin Effect Loss

Skin effect loss At a high frequency, current has a tendency
to concentrate around the surface rather than being
distributed throughout the cross section. This is known as
skin effect.
It reduces the effective surface area, which in turn
increases the resistance and hence the loss of the device.
Resistance loss is also proportional to the square of the
frequency.
Losses due to skin effect can be reduced by increasing the
current-carrying area, which, in turn, increases the inter-
electrode capacitance and thus limits high frequency
operations.

RF Loss- Dielectric Loss

RF Loss- Dielectric Loss
Dielectric loss Dielectric loss in a material is
proportional to frequency, and hence plays an
important role in the operations of high-frequency
tubes. This loss can be avoided by eliminating the
tube base and reducing the surface area of the
dielectric materials, and can be reduced by placing
insulating materials at the point of minimum electric
field.

Radiation Loss

Radiation Loss
Radiation loss At higher frequencies, the length of the
leads approaches the operating wavelength, and as a
result these start radiating. Radiation loss increases
with the increase in frequency and hence is very severe
at microwave frequencies. Proper shielding is required
to avoid this loss. Radiation loss can be minimized by
enclosing the tubes or using a concentric line
construction

Resonator
A resonator is a device or system that
exhibits resonance or resonant behavior, that is, it
naturally oscillates at some frequencies, called its
resonant frequencies, with greater amplitude than at
others.

Resonant Circuit

Resonant Circuit
An electrical circuit composed of discrete
components can act as a resonator when both
an inductor and capacitor are included.
Such resonant circuits are also called RLC
circuits after the circuit symbols for the components.

Cavity Resonator

Cavity Resonator
A cavity resonator, usually used in reference to
electromagnetic resonators, is one in which
waves exist in a hollow space inside the device

Cavity Resonator
Due to the low resistance of their conductive walls, cavity
resonators have very high Q factors; that is
their bandwidth, the range of frequencies around the
resonant frequency at which they will resonate, is very
narrow.
Thus they can act as narrow bandpass filters.
Cavity resonators are widely used as the frequency
determining element in microwave oscillators.
Their resonant frequency can be tuned by moving one of
the walls of the cavity in or out, changing its size.

Rectangular Cavity Resonator

For a > b < d, the dominant
mode is the TE101 mode.

The electric field lines start
from top and bottom, positive
and negative charges are
induced, hence forms
capacitor
The current flows via side
walls and hence serve as
inductor, hence the enclosed
volume behaves as tank
circuit.

Circular Cavity Resonator

Circular Cavity Resonator
TE111 mode is the dominant mode.

Quality Factor
The Q factor (quality factor) of a resonator is a
measure of the strength of the damping of its
oscillations, or for the relative linewidth.
the Q factor is 2π times the ratio of the stored
energy to the energy dissipated per oscillation
cycle
the Q factor is the ratio of the resonance
frequency ν0 and the full width at half-maximum
(FWHM)bandwidth δν of the resonance:

Quality Factor

Reentrant Cavity Resonator

Excitation Wave Modes
Loop coupling Probe coupling

Probe coupling

Loop coupling

Aperture Coupling
Aperture coupling

Coupling Between Waveguides
Directional Coupler

Linear Beam Tubes-Otype Tubes
Electric Field is applied to the accelerate
or decelerate the Electron beam
Magnetic Field is applied along the axis to
Focus the electron beam.

Klystron
an electron tube that generates or amplifies microwaves by velocity modulation.

Klystron- Velocity Modulation
Velocity of electrons accelerated by high DC Voltage

Gap Voltage applied at Buncher grids
Where

Average transit time through buncher gap

Average Voltage across the buncher gap

Equation for Velocity Modulation

Klystron- Bunching Process

Distance travelled by the electrons in drift space.

Klystron- Current Modulation
Beam Current varies with the applied RF voltage –
current modulation.
Fundamental component of current
Current becomes maximum at

Klystron- Current Modulation
Optimum distance for bunching

Klystron

Applegate Diagram

Output Power

Efficiency
Theoretical efficiency is 58%
Where as practical efficiency is 15% to 30%

Voltage Gain

Typical Values

Applications
As power output tubes
1. in UHF TV transmitters
2. in troposphere scatter
transmitters
3. satellite communication
ground station
4. radar transmitters

Klystron

Multi cavity Klystron

Reflex Klystron

Velocity Modulation
Velocity of the electrons in entering the cavity gap

Velocity Modulation
Exit Velocity of the electrons in leaving the cavity gap

Velocity Modulation
Retarding Electric Field

Velocity Modulation
Force equation of one electron assuming V1<<(Vr+Vo)

Reflex Klystron
Integrating

Reflex Klystron

Transit Time
Round trip transit time in the repeller region

Transit Time

Applegate Diagram

Efficiency

Efficiency of Reflex Klystron

Characteristics of Reflex Klystron

Electronic Admittance

Bunched electrons return to the cavity gap a little before
the transit time, current leads the behind the field-
capacitance appears in the circuit

Bunched electrons return to the cavity gap a little after to
The ac current lags the field –inductance reactance
appears in the circuit

Condition for oscillation Ge is negative and total
conductance in the circuit is negative –Ge>Gc+Gl

Applications
Low power oscillator- 10mw to 500mw
Frequency 1-25GHz
Local Oscillator in commercial , Military,
Air borne Doppler radar and missiles.

Tuning Klystron
Electronic Tuning

Tuning Klystron
Mechanical Tuning: By changing capacitance or inductance

Klystron

Klystron
Output is via a co-axial pin,
and the device can be
mechanically tuned with the
screw on the left, which
applies vertical compression
to the metal envelope.

Amplitude Modulation -Klystron

Frequency Modulation Klystron

Slow Wave Structures
Non Resonant periodic circuits
Produce large gain over wide bandwidth

Slow Wave Structures

Slow Wave Structure

Phase Velocity

Group Velocity

Travelling wave tube

Amplifiers in satellite transponders, where the
input signal is very weak and the output needs
to be high power.
TWTA transmitters are used extensively
in radar, particularly in airborne fire-control
radar systems, and in electronic warfare and
self-protection systems

Linear Beam tubes –O type
Klystron – Resonant , standing wave
Reflex Klystron- Resonant, standing wave
Travelling wave tube- Non resonant, travelling wave

Amplifies a wide range of frequencies, a wide bandwidth
and low noise.
Bandwidth two octaves, while the cavity versions have
bandwidths of 10–20%.
Operating frequencies range from 300 MHz to 50 GHz.
The power gain of the tube is on the order of 40 to
70 decibels
Output power ranges from a few watts to megawatts

Octave
A frequency is said to be an octave in width
when the upper band frequency is twice the
lower band frequency

Crossed Field tubes –M type
Crossed-field tubes derive their name from the
fact that the dc electric field and the dC magnetic
field are perpendicular to each other.
They are also called M –type tubes

Cylindrical Magnetron

Travelling wave Magnetron
Depend upon the interaction of electrons with a
rotating electromagnetic field of same angular
velocity.
Provide oscillations of very high peak power and
hence are useful in radar applications

Cavity Magnetron
Fig (i) Major elements in the Magnetron oscillator

Anode Assembly

Construction
 Each cavity in the anode acts as an inductor having only
one turn and the slot connecting the cavity and the
interaction space acts as a capacitor.
 These two form a parallel resonant circuit and its resonant
frequency depends on the value of L of the cavity and the
C of the slot.
 The frequency of the microwaves generated by the
magnetron oscillator depends on the frequency of the RF
oscillations existing in the resonant cavities.

Reentrant Cavity

Reentrant Cavity
E
B

Description
 Magnetron is a cross field device as the electric field
between the anode and the cathode is radial whereas the
magnetic field produced by a permanent magnet is axial.
 A high DC potential can be applied between the cathode
and anode which produces the radial electric field.
 Depending on the relative strengths of the electric and
magnetic fields, the electrons emitted from the cathode
and moving towards the anode will traverse through the
interaction space as shown in Fig. (iii).
 In the absence of magnetic field (B = 0), the electron travel
straight from the cathode to the anode due to the radial
electric field force acting on it, Fig (iii) a.

Cavity Magnetron

Description
 If the magnetic field strength is increased slightly, the
lateral force bending the path of the electron as given by
the path ‘b’ in Fig. (iii).
 The radius of the path is given by, If the strength of the
magnetic field is made sufficiently high then the electrons
can be prevented from reaching the anode as indicated
path ‘c’ in Fig. (iii)),
 The magnetic field required to return electrons back to the
cathode just grazing the surface of the anode is called the
critical magnetic field (Bc) or the cut off magnetic field.
 If the magnetic field is larger than the critical field (B > Bc),
the electron experiences a greater rotational force and may
return back to the cathode quite faster.

Fig (iii) Electron trajectories in
the presence of crossed electric
and magnetic fields
(a) no magnetic field
(b) small magnetic field
(c) Magnetic field = Bc
(d) Excessive magnetic field

e-
B
Fm
e-
B
Effect of electric field Effect of magnetic field
E
e-
Effect of Crossed-Fields

Hull Cut off Condition

Force due to magnetic field on charge Q moving with velocity v
Force on electron moving with velocity v

Force due to electric field on electron

Magnetic Field Bz az

Equations of electrons in motion
Acceleration due to electric field

Equations of Electrons in motion

Rearranging the equation (2)

Angular Velocity

Kinetic Energy of Electrons
Velocity of electrons

Hull Cutoff Magnetic Equation

Hull Cutoff Voltage Equation

Cyclotron Angular Frequency

Time Period

Phase shift between adjacent
cavities

Phase constant

π Mode

RF Field

PH0101 Unit 2 Lecture 5 235
Working
Fig (iv) Possible trajectory of electrons from cathode to anode in an eight cavity
magnetron operating in  mode

Working
 The RF Oscillations of transient nature produced when
the HT is switched on, are sufficient to produce the
oscillations in the cavities, these oscillations are
maintained in the cavities reentrant feedback which
results in the production of microwaves.
 Reentrant feedback takes place as a result of interaction
of the electrons with the electric field of the RF
oscillations existing in the cavities.
 The cavity oscillations produce electric fields which fringe
out into the interaction space from the slots in the anode
structure, as shown in Fig (iv).
 Energy is transferred from the radial dc field to the RF
field by the interaction of the electrons with the fringing
RF field.

Working
 Due to the oscillations in the cavities, the either sides of
the slots (which acts as a capacitor) becomes alternatively
positive and negative and hence the directions of the
electric field across the slot also reverse its sign
alternatively.
 At any instant the anode close to the spiraling electron
goes positive, the electrons gets retarded and this is
because; the electron has to move in the RF field, existing
close to the slot, from positive side to the negative side of
the slot.
 In this process, the electron loses energy and transfer an
equal amount of energy to the RF field which retard the
spiraling electron.
 On return to the previous orbit the electron may reach the
adjacent section or a section farther away and transfer
energy to the RF field if that part of the anode goes
positive at that instant.

Working
 This electron travels in a longest path from cathode to the
anode as indicated by ‘a’ in Fig (iv), transferring the
energy to the RF field are called as favoured electrons and
are responsible for bunching effect and give up most of its
energy before it finally terminates on the anode surface.
 An electron ‘b’ is accelerated by the RF field and instead
of imparting energy to the oscillations, takes energy from
oscillations resulting in increased velocity, such electrons
are called unfavoured electrons which do not participate in
the bunching process and cause back heating.
 Every time an electron approaches the anode “in phase”
with the RF signal, it completes a cycle. This corresponds
to a phase shift 2.
 For a dominant mode, the adjacent poles have a phase
difference of  radians, this called the  - mode.

Fig (v) Bunching of electrons in
multicavity magnetron

Working
 At any particular instant, one set of alternate poles
goes positive and the remaining set of alternate poles
goes negative due to the RF oscillations in the cavities.
 AS the electron approaches the anode, one set of
alternate poles accelerates the electrons and turns
back the electrons quickly to the cathode and the other
set alternate poles retard the electrons, thereby
transferring the energy from electrons to the RF signal.
 This process results in the bunching of electrons, the
mechanism by which electron bunches are formed and
by which electrons are kept in synchronism with the RF
field is called phase focussing effect. electrons with the
fringing RF field.

Working
 The number of bunches depends on the number of
cavities in the magnetron and the mode of oscillations, in
an eight cavity magnetron oscillating with  - mode, the
electrons are bunched in four groups as shown in Fig (v).
 Two identical resonant cavities will resonate at two
frequencies when they are coupled together; this is due to
the effect of mutual coupling.
 Commonly separating the pi mode from adjacent modes is
by a method called strapping. The straps consist of either
circular or rectangular cross section connected to alternate
segments of the anode block.

Performance Characteristics
1. Power output: In excess of 250 kW ( Pulsed
Mode), 10 mW (UHF band), 2 mW (X band),
8 kW (at 95 GHz)
2. Frequency: 500 MHz – 12 GHz
3. Duty cycle: 0.1 %
4. Efficiency: 40 % - 70 %

Applications of Magnetron
1. Pulsed radar is the single most important
application with large pulse powers.
2. Voltage tunable magnetrons are used in sweep
oscillators in telemetry and in missile
applications.
3. Fixed frequency, CW magnetrons are used for
industrial heating and microwave ovens.

Mode Jumping

Mode Jumping
Strapping Rising sun structure

Disadvantages
• They are costly and hence limited in use.
• Although cavity magnetron are used because
they generate a wide range of frequencies , the
frequency is not precisely controllable.
• The use in radar itself has reduced to some
extent, as more accurate signals have generally
been needed and developers have moved to
klystron and systems for accurate frequencies.

Cross Field Amplifier

The Crossed-Field Amplifier (CFA), is a broadband microwave amplifier that can also be
used as an oscillator (Stabilotron).
It is a so called Velocity-modulated Tube . The CFA is similar in operation to
themagnetron and is capable of providing relatively large amounts of power with high
efficiency.
In contrast to the magnetron, the CFA have an odd number of resonant cavities
coupled with each other. These resonant cavities work to as a slow-wave structure: an
oscillating resonant cavity excites the next cavity.
The actual oscillation will be lead from the input waveguide to the output waveguide.
The electric and magnetic fields in a CFA are perpendicular to each other (“crossed
fields”). Without an input signal and the influence of both the electric field (anode
voltage) and the magnetic field (a strong permanent magnet) all electrons will move
uniformly from the cathode to the anode on a cycloidal path as shown in figure

If the input-waveguide introduces an oscillation into the first resonator, the vanes of
the resonator gets a voltage difference synchronously to the oscillation.
Under the influence of this additionally field flying past electrons get acceleration (at
the positively charged vane) or they are decelerated (at the negatively charged vane).
This causes a difference in speed of the electrons. The faster electrons catch the slower
electrons and the forms electron bunches in the interaction space between the
cathode and the anode.
These bunches of electrons rotates as like as the “Space-Charge Wheel” known from
the magnetron operation. But they cannot rotate in full circle, the “Space-Charge
Wheel” will be interrupted because the odd number of cavities causes an opposite
phase in the last odd cavity (this bottom one between the waveguides).
To avoid a negative feedback, into this resonant cavity may exist a bloc containing
graphite to decouple input and output.

The bandwidth of the CFA, at any given instant, is approximately plus or minus 5 percent
of the rated center frequency.
Any incoming signals within this bandwidth are amplified. Peak power levels of many
megawatts and average power levels of tens of kilowatts average are, with efficiency
ratings in excess of 70 percent, possible with crossed-field amplifiers.
To avoid ineffective modes of operation the construction of CFA contains strapping
wires like to as used in magnetrons.
Because of the desirable characteristics of wide bandwidth, high efficiency, and the
ability to handle large amounts of power, the CFA is used in many applications in
microwave electronic systems.
When used as the intermediate or final stage in high-power radar systems, all of the
advantages of the CFA are used.

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